**4. Opportunities for the production of compliant gases**

In the previous sections, the conditions for injecting processed biogas into a natural gas grid were described in detail. In this chapter, appropriate mixture compositions matching the individual gas types will be determined and the possibilities for conditioning with air and / or propane / butane admixture will be discussed.

Four different methane volume fractions (processing grades) are shown, for a total of three natural gas types as a "target" properties. Depending on the application, LPG admixtures and air admixtures are applied across a large range in order to determine a suitable, practical combination.

The addition of liquid gas or air is to be understood as an additive to the processed biogas (100%). This means that the proportion of the total mixture (100% + X) is lower than the amount added. The depicted LPG addition shows the component added, the value of the Wobbe Index, calorific value and the limits of propane and butane components resulting from the total mixture.

$$\text{LMTPG in Gemish} = \frac{\dot{V}\_{\text{LPG Zuggable}}}{\dot{V}\_{\text{LPG Zuggable}} + \dot{V}\_{\text{Biggas}}} \tag{4}$$

The type of conditioning selected depends upon economic and technical factors and will ensure that the broadest possible spectrum of combustion values is achieved.

For implementation in practice, it should be noted that the methane volume fractions arising from processing may be subject to fluctuations. Equally, the composition of the LPG may vary and the measuring instruments and the control and regulating equipment will have tolerances, so that error propagation through the system needs to be noted when trying to achieve the desired bandwidth of "target" properties.

#### **4.1 Target properties: North sea I H gas**

For the production of compliant H gas with technical combustion characteristics matching North Sea I specifications, conditioning by admixing LPG is examined below.

The figures 2 to 5 show the potential composition of biogas mixtures, based upon methane levels in the processed bio-gas of 94, 96, 98 and 99,5 vol..-%, to which propane/butane (in a ratio of 95 / 5) is added. The necessary LPG admixtures for the desired calorific range for a methane content of 94, 96 and 98 vol -% in the treated biogas, lie above the limits as defined in G 486-B2 and DIN 51624, at 9.4 to 12.6 Vol -%, from 8.1 to 11.3 Vol -% and 6.8 to 9.9 vol -%. For processing to a methane content of 99.5 Vol -%, the limit according to G 486-B2 for pressures <100 bar is numerically satisfied up to a propane / butane admixture of 6.5 vol -%. The applicability criteria as described in section 2 apply. On the basis of this restriction, only an admixture of 5.8 to 6.5Vol.-% of LPG for an initial methane content of 99.5Vol.-% is possible. This would then cover a calorific range of 11.971 to 12.080 kWh/m³.

for the dew point temperatures lie below the requirements defined in G 260 (about 4 - 7 ° C

In the previous sections, the conditions for injecting processed biogas into a natural gas grid were described in detail. In this chapter, appropriate mixture compositions matching the individual gas types will be determined and the possibilities for conditioning with air and /

Four different methane volume fractions (processing grades) are shown, for a total of three natural gas types as a "target" properties. Depending on the application, LPG admixtures and air admixtures are applied across a large range in order to determine a suitable,

The addition of liquid gas or air is to be understood as an additive to the processed biogas (100%). This means that the proportion of the total mixture (100% + X) is lower than the amount added. The depicted LPG addition shows the component added, the value of the Wobbe Index, calorific value and the limits of propane and butane components resulting

*LPG Zugabe LPG im Gemisch*

The type of conditioning selected depends upon economic and technical factors and will

For implementation in practice, it should be noted that the methane volume fractions arising from processing may be subject to fluctuations. Equally, the composition of the LPG may vary and the measuring instruments and the control and regulating equipment will have tolerances, so that error propagation through the system needs to be noted when trying to

For the production of compliant H gas with technical combustion characteristics matching

The figures 2 to 5 show the potential composition of biogas mixtures, based upon methane levels in the processed bio-gas of 94, 96, 98 and 99,5 vol..-%, to which propane/butane (in a ratio of 95 / 5) is added. The necessary LPG admixtures for the desired calorific range for a methane content of 94, 96 and 98 vol -% in the treated biogas, lie above the limits as defined in G 486-B2 and DIN 51624, at 9.4 to 12.6 Vol -%, from 8.1 to 11.3 Vol -% and 6.8 to 9.9 vol -%. For processing to a methane content of 99.5 Vol -%, the limit according to G 486-B2 for pressures <100 bar is numerically satisfied up to a propane / butane admixture of 6.5 vol -%. The applicability criteria as described in section 2 apply. On the basis of this restriction, only an admixture of 5.8 to 6.5Vol.-% of LPG for an initial methane content of 99.5Vol.-% is possible. This would then cover a calorific range of 11.971 to 12.080

=

ensure that the broadest possible spectrum of combustion values is achieved.

North Sea I specifications, conditioning by admixing LPG is examined below.

*V*

*V V*

*LPG Zugabe Biogas*

+

(4)

(soil temperature at 1 m depth) at line pressure).

or propane / butane admixture will be discussed.

practical combination.

from the total mixture.

kWh/m³.

**4. Opportunities for the production of compliant gases** 

*x*

achieve the desired bandwidth of "target" properties.

**4.1 Target properties: North sea I H gas** 

Fig. 2. Possible H gas mixtures by admixing LPG to an initial concentration of 94 Vol. -% methane

Fig. 3. Possible H gas mixtures by admixing LPG to an initial concentration of 96 Vol. -% methane

Conditioning of Biogas for Injection into the Natural Gas Grid 381

Table 8 shows a summary of LPG additions necessary to achieve an average target calorific

For the production of compliant, low calorific L gas, conditioning by the addition of air is

Figures 6 to 9 inclusive show possible fuel gas mixtures with a calorific value range of 9.653

 **Luft Zugabe Wobbe - Index**

**-Konzentration**

value of approx.12.2 kWh/m³.

**Methane North Sea I** 

Vol.-% Vol.-% 94 11,12 96 9,72 98 8,32 99,5 7,34

**4.2 Target properties: Weser ems L gas** 

described in the following sections.

 **Methan**

concentration of 94 Vol. -% methane

**Methan-Konzentration** ϕ**CH4**

 **in Vol.-%**

**After processing HS,n = 12,2 kWh/m³** 

to 10.047 kWh / m³, which can be achieved by the addition of air.

Table 8. LPG quantities necessary to achieve the average target calorific value

 **O2**

**8,6 8,8 9,0 9,2 9,4 9,6 9,8 10,0 10,2 10,4 10,6 10,8 11,0**

**HS,n in kWh/m³**

Fig. 6. Possible low calorific L gas mixtures achieved by admixing air to an initial

4 Exceeds the maximum concentration of propane for p < 100 bar according to G 486-B2

**0**

**10,5**

**0,0**

**0,5**

**1,0**

 **Sauerstoff-Konzentration** ϕ

**1,5**

**2,0**

**O2 in Vol.-%**

**2,5**

**3,5**

**4,0**

**3%-Grenze**

**11,0**

**11,5**

**12,0**

**Wobbe - Index WS,n in kWh/m³**

**12,5**

**13,5**

**14,0**

**14,5**

**15,0**

**L-Gas Grenze**

**2**

**4**

**3,6**

**6**

**8**

**7,7**

**Luft Zugabe in Vol.-%**

**10**

**12**

**14**

**16**

**18**

**20**

Fig. 4. Possible H gas mixtures by admixing LPG to an initial concentration of 98 Vol. -% methane

Fig. 5. Possible H gas mixtures by admixing LPG to an initial concentration of 99,5 Vol. -% methane

 **LPG Zugabe Wobbe-Index**

10,0 10,5 11,0 11,5 12,0 12,5 13,0 13,5 14,0 14,5

**HS,n in kWh/m³**

 **LPG Zugabe**

10,0 10,5 11,0 11,5 12,0 12,5 13,0 13,5 14,0 14,5

**HS,n in kWh/m³**

Fig. 5. Possible H gas mixtures by admixing LPG to an initial concentration of 99,5 Vol. -%

Fig. 4. Possible H gas mixtures by admixing LPG to an initial concentration of 98 Vol. -%

 **Wobbe - Index**

 **relative Dichte**

13,0

0,56 0,58 0,60 0,62 0,64 0,66 0,68 0,70 0,72 0,74

**relative Dichte**

13,5

14,0

14,5

**Wobbe - Index WS,n in kWh/m³**

**LPG-Zugabe in Vol.-%**

**8,9**

**5,8**

15,0

15,5

16,0

**H-Gas Grenze**

16,5

13,0

0,56 0,58 0,60 0,62 0,64 0,66 0,68 0,70 0,72 0,74

**relative Dichte**

13,5

14,0

14,5

**Wobbe - Index WS,n in kWh/m³**

15,0

**LPG-Zugabe in Vol.-%**

**6,8**

**9,9**

 **relative Dichte**

15,5

16,0

**H-Gas-Grenze**

16,5

 **Methan**

methane

**Methan-Konzentration** ϕ**CH4**

 **in Vol.-%**

methane

**Methan-Konzentration** ϕ**CH4 in Vol.-%**

 **Methan**

Table 8 shows a summary of LPG additions necessary to achieve an average target calorific value of approx.12.2 kWh/m³.


Table 8. LPG quantities necessary to achieve the average target calorific value

### **4.2 Target properties: Weser ems L gas**

For the production of compliant, low calorific L gas, conditioning by the addition of air is described in the following sections.

Figures 6 to 9 inclusive show possible fuel gas mixtures with a calorific value range of 9.653 to 10.047 kWh / m³, which can be achieved by the addition of air.

Fig. 6. Possible low calorific L gas mixtures achieved by admixing air to an initial concentration of 94 Vol. -% methane

<sup>4</sup> Exceeds the maximum concentration of propane for p < 100 bar according to G 486-B2

Conditioning of Biogas for Injection into the Natural Gas Grid 383

The higher the initial content of methane in the biogas is, the greater is the approximation to

Reaching of the required calorific value band (9.653 to 10.047 kWh / m³) is possible from all

Figure 8 shows a summary of the air admixture ranges of the four initial methane levels. The red line indicates the maximum permissible volume fraction of 3% of O2 in the mixture.

Niedrigkalorisches L-Gas: Zusammensetzungen für einen Brenwertbereich von Hs,n=9,653-10,047 kWh/m<sup>3</sup>

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Luft-Zumischrate [Vol.-%]

**96 Vol.-% Methan**

**94 Vol.-% Methan**

Fig. 8. Rates of air admixture at different initial concentrations of methane

**98 Vol.-% Methan**

**99,5 Vol.-% Methan**

94,0 5,6 12,379 96,0 7,8 12,492 98,0 10,1 12,589 99,5 11,8 12,664 Table 9. Air admixture to Weser Ems L-Gas and corresponding Wobbe-Index

the maximum compliant O2 content of 3 vol -% from conditioning. Thus the O2 levels upon reaching the lower calorific value band are:


Weser Ems L Gas HS,n = 9,85 kWh/m³ Air added to the biogas Wobbe Index in Vol.-% in kWh/m³

Table 9 shows the respective admixtures.

Methane concentration after processing in vol -%

four initial methane contents.

Figure 6 shows the possible mixture compositions, when air is added to an initial methane content of 94 vol -% . When interpreting this, please note that the data points for the Wobbe index, the methane content and the air belong together along the line of constant calorific value. A +/- 2% range has been set for the calorific value limits, based on the calorific value defined in DVGW worksheet G 260. All values with a Wobbe index of less than 13 kWh / m³ and an O2 content of less than 3 vol -% meet requirements. All other boundary conditions are shown in the table above.

Figure 7 show the possible mixture compositions, when air is added to initial methane content of 99,5 Vol.-%.

The two cases presented with initial methane contents of 94 and 99.5 Vol -% in the biogas, clearly show that increases in methane content also make necessary increased amounts of air, in order to achieve the desired calorific value and Wobbe index.

Fig. 7. Possible low calorific L gas mixtures achieved by admixing air to an initial concentration of 99,5 Vol. -% methane


Table 9 shows the respective admixtures.

382 Biogas

Figure 6 shows the possible mixture compositions, when air is added to an initial methane content of 94 vol -% . When interpreting this, please note that the data points for the Wobbe index, the methane content and the air belong together along the line of constant calorific value. A +/- 2% range has been set for the calorific value limits, based on the calorific value defined in DVGW worksheet G 260. All values with a Wobbe index of less than 13 kWh / m³ and an O2 content of less than 3 vol -% meet requirements. All other boundary conditions

Figure 7 show the possible mixture compositions, when air is added to initial methane

The two cases presented with initial methane contents of 94 and 99.5 Vol -% in the biogas, clearly show that increases in methane content also make necessary increased amounts of

 **Wobbe-Index**

air, in order to achieve the desired calorific value and Wobbe index.

 **Methan Luft Zugabe O2**

**8,6 8,8 9,0 9,2 9,4 9,6 9,8 10,0 10,2 10,4 10,6 10,8 11,0**

Fig. 7. Possible low calorific L gas mixtures achieved by admixing air to an initial

**HS,n in kWh/m³**

**Luft Zugabe in Vol.-%**

**0,0**

**10,5**

**11,0**

**11,5**

**12,0**

**12,5**

**13,5**

**14,0**

**14,5**

**15,0**

**L-Gas Grenze**

**Wobbe - Index WS,n in kWh/m³**

**0,5**

**1,0**

 **Sauerstoff-Konzentration** ϕ**O2 in**

 **Vol.-%**

**1,5**

**2,0**

**2,5**

**14,0**

**9,7**

**3,5**

**4,0**

**-Konzentration**

**3%-Grenze**

are shown in the table above.

content of 99,5 Vol.-%.

concentration of 99,5 Vol. -% methane

**Methan-Konzentration** ϕ**CH4 in**

 **Vol.-%** Table 9. Air admixture to Weser Ems L-Gas and corresponding Wobbe-Index

The higher the initial content of methane in the biogas is, the greater is the approximation to the maximum compliant O2 content of 3 vol -% from conditioning.

Thus the O2 levels upon reaching the lower calorific value band are:


Reaching of the required calorific value band (9.653 to 10.047 kWh / m³) is possible from all four initial methane contents.

Figure 8 shows a summary of the air admixture ranges of the four initial methane levels. The red line indicates the maximum permissible volume fraction of 3% of O2 in the mixture.

Fig. 8. Rates of air admixture at different initial concentrations of methane

Conditioning of Biogas for Injection into the Natural Gas Grid 385

8,8 9,2 9,6 10,0 10,4 10,8 11,2 11,6 12,0 12,4 12,8 13,2 13,6 14,0

8,8 9,2 9,6 10,0 10,4 10,8 11,2 11,6 12,0 12,4 12,8 13,2 13,6 14,0

HS,n in kWh/m³

8,8 9,2 9,6 10,0 10,4 10,8 11,2 11,6 12,0 12,4 12,8 13,2 13,6 14,0

8,8 9,2 9,6 10,0 10,4 10,8 11,2 11,6 12,0 12,4 12,8 13,2 13,6 14,0

**O2**

18

**< 3% Grenze**

HS,n in kW h/m³

Fig. 11. Possible highly calorific L gas mixtures by admixing air and LPG to an initial

12 14 16

Fig. 10. Possible highly calorific L gas mixtures by admixing air and LPG to an initial

LPG-Zugabe

**O2**

**< 3% Grenze**

10,0 10,5 11,0 11,5 12,0 12,5

10,0 10,5 11,0 11,5 12,0 12,5

L-Gas Grenze

13,5 14,0 14,5 15,0 15,5 16,0

0

4 6 8 10 12 14 16 18 <sup>20</sup> <sup>20</sup>

2

Luft - Zufuhr

L-Gas Grenze

13,5 14,0 14,5 15,0 15,5 16,0

0 2 4 6 8 10 12 14 16 18 20

Luft Zugabe

10,0 10,5 11,0 11,5 12,0 12,5 13,0 13,5 14,0 14,5 15,0 15,5 16,0

10,0 10,5 11,0 11,5 12,0 12,5 13,0 13,5 14,0 14,5 15,0 15,5 16,0

W

in kWh/m³

S,n

WS,nin kWh/m³

2 4 6 8 10 12 14 16 18 20

0

concentration of 96 Vol. -% methane.

0 2 4 6 8 10

concentration of 98 Vol. -% methane

LPG - Zufuhr

#### **4.3 Target properties: Holland II L gas**

For the production of compliant, high calorific L gas, conditioning by the addition of air and LPG is described in the following section.

Please note the following when interpreting the diagrams below: The field of admixtures includes a range of 0 - 20 Vol -% for presentational purposes. In practice, for technical and economic reasons, it is desirable to make the least possible admixtures with a "target" Wobbe Index of 12.4 kWh / m³ for example (setting of the gas appliances). In this context it should be noted that according to G 486 appendix B, the mole fractions of propane are not to exceed 3.5 mol% (6 mol% at p <100 bar) and butane max.1.5 mol% in natural gas, in order make a conversion of standard and operating conditions using the AGA8-DC92 equation of state.

The "field" of the possible mixtures is bounded by the Wobbe Index of 13 kWh / m³, the given calorific value limits, the max. oxygen volume fraction of 3%, and the maximum propane/butane or air admixture. For each value of air addition, there is always a value for the propane / butane addition.

The following figures apply only to the four initial properties of the biogas used.

Figures 9 and 10 show the calorific values and the Wobbe index for an air and LPG admixture of 0 to 20 vol -% to a biogas with an initial methane content of 94 vol -% and 96 vol -%.

Fig. 9. Possible highly calorific L gas mixtures by admixing air and LPG to an initial concentration of 94 Vol. -% methane.

For the production of compliant, high calorific L gas, conditioning by the addition of air and

Please note the following when interpreting the diagrams below: The field of admixtures includes a range of 0 - 20 Vol -% for presentational purposes. In practice, for technical and economic reasons, it is desirable to make the least possible admixtures with a "target" Wobbe Index of 12.4 kWh / m³ for example (setting of the gas appliances). In this context it should be noted that according to G 486 appendix B, the mole fractions of propane are not to exceed 3.5 mol% (6 mol% at p <100 bar) and butane max.1.5 mol% in natural gas, in order make a conversion of standard and operating conditions using the AGA8-DC92 equation of

The "field" of the possible mixtures is bounded by the Wobbe Index of 13 kWh / m³, the given calorific value limits, the max. oxygen volume fraction of 3%, and the maximum propane/butane or air admixture. For each value of air addition, there is always a value for

Figures 9 and 10 show the calorific values and the Wobbe index for an air and LPG admixture of 0 to 20 vol -% to a biogas with an initial methane content of 94 vol -% and 96

8,8 9,2 9,6 10,0 10,4 10,8 11,2 11,6 12,0 12,4 12,8 13,2 13,6 14,0

8,8 9,2 9,6 10,0 10,4 10,8 11,2 11,6 12,0 12,4 12,8 13,2 13,6 14,0

HS,n in kWh/m³

Fig. 9. Possible highly calorific L gas mixtures by admixing air and LPG to an initial

LPG-Zufuhr

12 14 16 18

**O2**

20

20

**< 3% Grenze**

14 16 18

> 10,0 10,5 11,0 11,5 12,0 12,5

L-Gas Grenze

13,5 14,0 14,5 15,0 15,5 16,0

2 4 6 8 10 12

Luft-Zufuhr

0

The following figures apply only to the four initial properties of the biogas used.

**4.3 Target properties: Holland II L gas** 

LPG is described in the following section.

the propane / butane addition.

state.

vol -%.

10,0 10,5 11,0 11,5 12,0 12,5 13,0 13,5 14,0 14,5 15,0 15,5 16,0

WS,n in kWh/m³

0 2 4 6 8 10

concentration of 94 Vol. -% methane.

Fig. 10. Possible highly calorific L gas mixtures by admixing air and LPG to an initial concentration of 96 Vol. -% methane.

Fig. 11. Possible highly calorific L gas mixtures by admixing air and LPG to an initial concentration of 98 Vol. -% methane

51624, 2008).

Siederdissen & Wundram, 1986).

are correspondingly smaller.

Initial

in Vol.-%

concentration of CH4 in the biogas

Conditioning of Biogas for Injection into the Natural Gas Grid 387

operation of modern engines, a methane number of MZ > 70 is considered necessary (DIN

For multi-component mixtures such as natural gases, the condensation and boiling curves do not lie together, but span a conditional area, where different gas-liquid compositions are possible. Between the critical pressure and the cricondenbar point with increasing temperature, and between the critical temperature and the criconden therm point with falling pressure, condensate (retrograde condensation) can form when the throttle curve touches the dew line, intersects or the final state lies in the two-phase region (Höner zu

An admixture of propane / butane to natural gas and processed biogas generally manifests itself in a shift of the dew curve to higher temperatures. According to (Oellrich et al., 1996), in the case of Russian H gas, condensation is only to be expected at temperatures of -35 ° C, while it will occur with Dutch L gas already at -5 ° C. If liquid gas/air is admixed within the limits described in DVGW worksheet G 260, the criconden therm point moves toward +15 ° C or +45 ° C, but at higher pressures. For mixtures of natural gas and processed conditioned biogas, this is to be expected to a lesser degree, since the concentrations of propane / butane

It should be noted that the calculation of the condensation curves of natural gases requires an analysis that takes into account the higher hydrocarbons, since even small amounts in the ppm range result in a significant shift. Furthermore, the process of condensation is not in itself critical, but the quantity of condensate is the decisive criterion. For large flow rates, a seemingly low volume of condensate can therefore lead to problems (Oellrich et al., 1996).

The following Table 10 and Figure 13 show cases of condensation in conditioning by the addition of LPG, in order to meet the North Sea I specification. The lowest and the highest admixtures were selected for the diagrams. It should be noted that at the highest level of

> Calorific value in kWh/m³

94,000 9,400 11,960 14,339 0,696 71 94,000 12,600 12,432 14,642 0,721 67 96,000 8,100 11,965 14,654 0,667 72 96,000 11,300 12,442 14,946 0,693 67 98,000 6,800 11,970 14,998 0,637 73 98,000 9,900 12,438 15,268 0,664 67 99,500 5,800 11,971 15,276 0,614 74 99,500 8,900 12,443 15,534 0,642 67 Table 10. Cases of condensation for mixtures of the North See I H gas specification

Wobbe Index in kWh/m³

rel. Density Methane number

admixing, the restrictions imposed by G 486 were not observed.

LPG addition to

biogas in Vol.-%

Figures 11 and 12 show the calorific values and the Wobbe index for an air and LPG admixture of 0 to 20 vol -% with an initial methane content of 98 vol -% and 99,5 vol -%.

Fig. 12. Possible highly calorific L gas mixtures by admixing air and LPG to an initial concentration of 99,5 Vol. -% methane

The red area represents the required calorific value range from 9.97 to 10.4 kWh / m³. The green dots show the possible, compliant mixtures that lie within all the conditions to be fulfilled.
